The present invention relates to an improved washer-type or Belleville spring, and more particularly to a Belleville spring having an improved fatigue life and surface residual compressive stresses.
Springs of the Belleville type have been employed in friction clutches and similar mechanical applications, such springs having a conical or roughly conical shape with an inner periphery which is normally spaced axially from the plane of the outer spring periphery and approaches that plane as the spring is placed under compression.
Presently known Belleville springs are formed of a plain carbon steel with standard alloy additions and a carbon content of about 0.60 to 1.00% in a cold rolled sheet or strip. To form a Belleville spring from this raw material, the basic shape of the spring is stamped or punched out of the steel strip and press formed into a truncated cone shape. After stamping and forming, the spring is heat treated at a temperature in the range of from about 1,400.degree. to 2,000.degree.F. and subsequently quenched in oil, water, salt or air to a temperature in the range of from about room temperature to 400.degree.F. This quenching may be followed by additional heating and cooling steps for tempering to relieve distortions in the material caused by quenching, to obtain the desired hardness and to set the spring to final shape.
This present production sequence is costly and requires additional preliminary operations due to the quality of the material utilized. The production of the high carbon, coldrolled sheet or strip steel is costly because the initial high carbon content results in a high strength and a strain hardening coefficient of the steel which necessitates multiple annealing and pickling operations during rolling of the raw material to final thickness. Also, the forces to stamp or punch out the basic spring shape are high due to the elevated strength of the high carbon steel which requires very strong and wear resistant dies.
Due to the rapid strain hardening of high carbon material, the edges of the stamped parts can become very brittle unless special precautions are taken. This brittleness may result in minute cracks at the edges which have to be removed by costly and time consuming methods, such as tumbling, grinding or the like, in order to avoid deleterious effects on the performance, particularly the fatigue life, of the finished Belleville springs. Similarly, the forming of the cone configuration of the Belleville spring in special dies may lead to the formation of sub-microscopic cracks in the high carbon, easily embrittled material.
The basic standard material used for the production of Belleville springs does not have a deliberately produced carbon gradient, but usually has a lower surface carbon content due to decarburization during the rolling and annealing operations of the raw material. The level of carbon determines the transformation temperature at which austenite transforms into martensite; the desired final structure. Thus, if the carbon level at the surface of the spring is less than the internal carbon level, the transformation temperature of the surface of the spring would be in a higher range than the transformation temperature range of the core. During quenching, a temperature gradient is produced in the Belleville spring resulting in a significantly lower surface temperature compared with the core temperature. This temperature gradient depends on the thickness and geometry of the material, but it can never be reversed during quenching. As a consequence of the inadvertently produced carbon gradient in the standard spring material and the temperature gradient during quenching, the surface of the belleville spring will transform into martensite earlier than the core.
The transformation of austenite into martensite results in a volume increase in the metal of 3% to 4%. Further, the resulting martensite structure is very strong but also extremely brittle as compared to austenite, which is fairly ductile. Consequently, during quenching, a very hard brittle layer of martensite will form on the surface of the spring so treated while the core is still in the austenitic state. The stresses set up during the transformation of the surface due to the volume increase will compress the core and deform the relatively ductile austenite therein. As soon as the core reaches the transformation temperature, it will be transformed into martensite and try to expand in volume against the resistance of the previously formed martensitic surface "shell".
This process obviously creates extremely high tensile stresses in the surface shell and leads to the generation of high residual tensile stresses therein. Very frequently, these tensile stresses become so high that the surface shell is cracked or even broken. Weaknesses or cracks so produced in the surface shell cannot be repaired by any known means; however, they can be camouflaged and to a certain extent counteracted by subsequent cold working of the spring surface by methods such as shot peening, vibratory finishing, ball peening, rolling, etc. In consequence of the above considered standard production sequence for presently manufactured Belleville springs, these springs have an inherent weakness resulting from the carbon and temperature distribution during quenching; this inherent weakness resulting in a comparatively low fatigue life.